1. Field of the Invention
This invention relates to a method and apparatus for servo motor drive control for high-accuracy positioning of a servo system such as an industrial robot.
2. Description of the Prior Art
A conventional servo motor control is described in an article entitled "Design and Analysis of Pulse-width-Modulated Amplifiers for DC Servo Systems" by Jacobtal, IEEE Transaction on Electronics and Control Instrumentation, Vol. IECI-23, No. 1, February 1976. Generally, a d.c. motor servo system has a motor driver stage as shown in FIG. 8. The driver stage consists of a pair of power control devices A and A' and another pair of power control devices B and B', the devices being typically bipolar transistors or FETs. Devices A and B have their collectors connected to the positive terminal of a servo motor power supply 12 and their emitters connected to the collectors of the devices B' and A respectively, with the emitters thereof connected to the negative terminal of the power supply 12 through a current detecting resistor R.sub.I. Each of the devices A and B is shunted between the collector and emitter terminals by flyback diodes A.sub.D and B.sub.D, respectively, which are polarized oppositely with respect to the emitter-collector junction. Similarly, the devices B' and A' have their collector terminals connected through opposite-polarized flyback diodes B.sub.D ' and A.sub.D ', respectively, to the negative terminals of the power supply 12. A d.c. servo motor 14, with its winding inductance and resistance represented by L.sub.M and R.sub.M, is connected between the emitter terminals of the power control devices A and B. This driver stage configuration in which two pairs of series-connected power control devices A--A' and B--B' are connected in parallel across a power source, with a d.c. servo motor 14 being connected between the node of A and B' and the node of B and A', is called "H-bridge" configuration. The four power control devices A, B, A' and B' in the H-bridge configuration have their base terminals supplied with input signals in pulse-width modulation (will be termed simply "PWM") from a servo control stage, and the current conduction to the d.c. servo motor 14 is controlled.
In the conventional d.c. servo motor drive control system, the device input signals are switched such that devices A and A' or devices B and B' are activated during the active cycle of the PWM signal, and the devices A and B are deactivated and the devices A' and B' are activated during the inactive cycle of the PWM signal. With the PWM signal having an active time length of t.sub.p and a PWM period of T.sub.p, the average application voltage V.sub.M to the servo motor 14 is given by the ordinary direct current theory as follows. EQU V.sub.M .apprxeq.V.multidot.t.sub.p /T.sub.p ( 1)
Examples of this type of d.c. motor drive circuit are disclosed in U.S. Pat. Nos. 4,388,570 and 4,523,134.
When high-accuracy positioning control is intended using a d.c. servo motor, the PWM signal needs to have an enhanced resolution as implied by the above formula (1). Namely, the PWM signal must have a very small minimum unit of active time length t.sub.p. On this account, it is necessary for the conventional servo motor drive control system intended for high-accuracy positioning control to have a large-scale circuitry for the PWM signal generation thereby to accomplish a fine resolution of the active time length t.sub.p, which is accompanied by circuitries for preventing erroneous operation and oscillation, resulting in an increased manufacturing cost. Moreover, the required PWM resolution will not be attained in some cases due to the limited response of the power control device and overall circuit.